The invention relates generally to an arrangement for mounting gas turbine nozzles and more specifically to an outer sidewall retention scheme for a singlet first stage nozzle.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer sidewall portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer sidewalls. A nozzle retaining ring coupled to the outer sidewall of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner sidewall and may support the first-stage nozzles against axial movement.
Side seals may seal the annular array of segments one to the other along adjoining circumferential edges. The side seals seal between a high pressure region radially inwardly of the inner sidewall and radially outward of the outer sidewall, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure. Chordal hinge seals are used to seal between the inner sidewall of the first-stage nozzles and an axially facing surface of the nozzle support ring and between the outer sidewall and a shroud for the first stage bucket.
Referring now to FIG. 1, there is illustrated a representative example of a generalized turbine section of a gas turbine, designated 10. Turbine 10 receives hot gases of combustion from an annular array of combustors, not shown, which transmit the hot gases through a transition piece 12 for flow along an annular hot gas path 14. Turbine stages are disposed along the hot gas path 14. Each stage comprises a plurality of circumferentially spaced buckets mounted on and forming part of the turbine rotor and a plurality of circumferentially spaced stator vanes forming an annular array of nozzles. For example, the first stage includes a plurality of circumferentially-spaced buckets 16 mounted on a first-stage rotor wheel 18 and a plurality of circumferentially-spaced stator vanes 20. Similarly, the second stage includes a plurality of buckets 22 mounted on a rotor wheel 24 and a plurality of circumferentially-spaced stator vanes 26. Additional stages may be provided, for example, a third stage comprised of a plurality of circumferentially-spaced buckets 28 mounted on a third-stage rotor wheel 30 and a plurality of circumferentially-spaced stator vanes 32. It will be appreciated that the stator vanes 20, 26 and 32 are mounted on and fixed to a turbine casing, while the buckets 16, 22 and 28 and wheels 18, 24 and 30 form part of the turbine rotor. Between the rotor wheels are spacers 34 and 36, which also form part of the turbine rotor. It will be appreciated that compressor discharge air is located in a region 37 disposed radially inwardly and radially outward of the first stage and that such air in region 37 is at a higher pressure than the pressure of the hot gases flowing along the hot gas path 14.
Referring to the first stage of the turbine, the stator vanes 20 forming the first-stage nozzles are disposed between inner and outer sidewalls 38 and 40, respectively, supported from the turbine casing. As noted above, the nozzles of the first stage are formed of a plurality of nozzle segments each mounting one, or two, stator vanes extending between inner and outer sidewall portions and arranged in an annular array of segments. A nozzle retaining ring 42 connected to the turbine casing is coupled to the outer sidewall and secures the first-stage nozzle. A nozzle support ring 44 radially inwardly of the inner sidewall 38 of the first-stage nozzles engages the inner sidewall 38. Particularly, the interface between the inner sidewall 38 and the nozzle support ring 44 includes an inner rail 52. The inner rail 52 includes a chord-wise, linearly extending axial projection, generally and collectively hereinafter referred to as a chordal hinge seal. It will be appreciated that high pressure compressor discharge air lies in the region 37 and lower pressure hot gases flowing in the hot gas path 14 lay on the opposite side of the chordal hinge seal. The chordal hinge seal is thus intended to seal against leakage from the high pressure region 37 into the lower pressure region of the hot gas path 14.
A nozzle comprises a plurality of radially extending airfoils arranged circumferentially about an engine axis, the airfoils being supported by radially inner and outer circumferential sidewalls. Either the inner or outer sidewalls may include some form of flange for coupling the nozzle to a stationary engine mounting structure. In general, a plurality of turbine nozzles is interleaved with a plurality of turbine rotor stages. The directing process performed by the nozzles also accelerates gas flow resulting in a static pressure reduction between inlet and outlet planes and high pressure loading of the nozzles. Additionally, the nozzles experience high thermal gradients from the hot combustion gases and the coolant air at the radial mounting surfaces.
The use of bolts and clamps at circumferential locations about a nozzle sidewall act as a restriction to the sidewall, which sidewall is hotter than the structure to which it is attached, causing radial bowing of the outer sidewall of the nozzle and stressing of the airfoils attached to the sidewall. Such stressing of the airfoils may lead to formation of cracks in the airfoil trailing edge.
FIG. 2 illustrates in greater detail a prior art sidewall retention system 100 for a first stage nozzle 110. The first stage nozzle 110 includes an outer sidewall 115, an inner sidewall 120 and an airfoil 125 positioned between a nozzle retaining ring 130 and a nozzle support ring 135. The nozzle retaining ring 130 and the support ring 135 are attached to the casing of the turbine (not shown). The first stage nozzle also includes chordal hinge rails for the inner sidewall and outer sidewall. The chordal hinge rail 145 on the inner sidewall 120 provides axial support for the nozzle 110 against the support ring 135 and the chordal hinge rail 150 provides axial support for the nozzle 110 against the shroud 160 of the first stage bucket 170. The inner chordal hinge rail 145 and outer chordal hinge rail 150 further provide chordal hinge seals 147, 152.
The chordal hinge rail 150 on the outer sidewall 115 of the nozzle 110 projects outward radially from the outer sidewall 115. The chordal hinge rail 150 incorporates a forward-facing annular retaining land 175 at its outermost radial projection. The retaining land 175 mates with an aft-facing annular groove 180 established by an aft-facing retaining hook 185 on the retaining ring. The retaining land 175 of the chordal hinge rail 150 acting on the retaining hook 185 of the retaining ring 130 provides radial support for the nozzle 110. The annular retaining hook 185 may be divided into segments (not shown). Circumferential support is provided by an anti-rotation pin (not shown) that passes through the retaining ring 130 and the retaining land 175.
Power generation gas turbines traditionally use some type of hook retention scheme. Improvements have been made on the traditional hook retention scheme by changing from a continuous hook arrangement, typical in FA class machines by GE to a segmented hook arrangement, typical in FB class machines by GE. This change resulted in more determinate nozzle loading and better nozzle sealing but also resulted in less than optimal thermal isolation of the retaining ring and thereby a substantial cost increase to the nozzle arrangement. Some of the field issues related to hook retention designs include incomplete chordal hinge sealing, retaining ring out of roundness, and high trailing edge stresses.
Accordingly, there is a need to provide determinate nozzle loading and improved sealing while also improving thermal isolation of the retaining ring, reducing cost, and improving assembly flexibility of the nozzle arrangement.